This invention relates generally to switch assemblies including miniaturized switch assemblies which are frequently employed in keyboards for controlling electronic equipment involving low voltage and low current use application.
A multitude of keyboard applications exist wherein keys or push buttons on a keyboard effect actuation of switches that are integrated into electronic circuits for carrying out a variety of computer or data processing operations. These range from keyboard applications in pocket calculators to both local and remote computer terminals. The cost of such keyboards is indeed a significant factor when the substantial number of terminals, input devices and other peripheral equipment uses in computer and data processing applications are considered. Further, large volume demand for miniaturized electrical switches occurs in solid state adding machines and calculators embracing desk top models ranging down to small pocket size calculators.
A variety of switch constructions suitable for keyboards of the types mentioned above have been developed in a wide range of both mechanical-electrical configurations and elaborate electronic switching concepts. Particularly, in the miniaturized switches needed for the very small and now low cost pocket size calculators, the large volume of keyboards needed makes cost in the keyboard construction a critical factor. Thus, the miniaturized switches must involve a low initial investment, be maintenance free in operation and possess high operational reliability. Meeting all of these criteria has indeed been a major problem in the development of miniaturized electrical switches.
One type of switch assembly suitable for keyboard type applications is described in U.S. Pat. No. 4,433,317, issued to the same applicant as the instant invention. The preferred embodiments disclosed in such patent provide for a pair of switch contact members formed of rod-like members that are precisely mounted in a housing cavity wherein the ends of the contact members are spaced to provide opposed contact surfaces. The contact surfaces are defined to be within an area bounded by the periphery of the contact rod member ends. A floating contactor member is disposed between the contact surfaces. The contactor member and contact rod ends are enclosed by a resilient sleeve which protects the switch from contamination, provides a centering, or return, force to maintain the switch in an open condition, and absorbs excessive operating forces. Operating forces are applied through the resilient sleeve by a trigger element associated with the switch housing.
The preferred embodiments disclosed in U.S. Pat. No. 4,433,317 satisfy many of the objectives of a miniaturized switch for keyboard-type applications. However, because the contact surfaces are within an area bounded by the periphery of the ends of the contact rods, the contact surfaces and the floating contactor member must be situated relative to one another in a precise manner. For example, the alignment between the opposed contact members is relatively critical, as is the spacing between these contact surfaces. Moreover, the contactor member configuration, such as the floating ball contactor, must be precisely manufactured. Deviations from the relatively precise design requirements will result in a switch that either shorts prematurely or is incapable of closing in its intended manner. Such precise manufacturing and assembly adds to the cost of the finished product.
Moveover, since the contact surfaces of the switch embodiments disclosed in U.S. Pat. No. 4,433,317, are defined to be within an area bounded by the periphery of the contact members, such as within the periphery of the opposed faces of the contact member rod ends, the contact member must be of relatively substantial diameter to permit shaping of the contact surfaces (such as the preferred conical shape). As such, the contact members are rod-like, as opposed to relatively thin wire, which adds to the cost of manufacture. Further, such rod-like members may transmit accidental forces that may be applied, for example, to the terminal ends, which may distort the relative spacing between the contact surfaces and the contactor member.
Still further, the preferred embodiments disclosed in U.S. Pat. No. 4,433,317, include a trigger element that is preferably formed of a relatively hard molded plastic and which includes a pressure finger to provide a force to the resilient sleeve at a relatively localized area on the sleeve. This may result in undesirable sleeve wear.